tuned signature;

--- a/src/Pure/Admin/build_cygwin.scala	Fri May 07 16:49:08 2021 +0200
+++ b/src/Pure/Admin/build_cygwin.scala	Fri May 07 21:03:20 2021 +0200
@@ -52,7 +52,7 @@
 
         (cygwin + Path.explode("Cygwin.bat")).file.delete
 
-        val archive = "cygwin-" + Date.Format("uuuuMMdd")(Date.now()) + ".tar.gz"
+        val archive = "cygwin-" + Date.Format.alt_date(Date.now()) + ".tar.gz"
         Isabelle_System.gnutar("-czf " + Bash.string(archive) + " cygwin", dir = tmp_dir).check
       })
   }

--- a/src/Pure/Admin/build_fonts.scala	Fri May 07 16:49:08 2021 +0200
+++ b/src/Pure/Admin/build_fonts.scala	Fri May 07 21:03:20 2021 +0200
@@ -358,7 +358,7 @@
       val more_args = getopts(args)
       if (more_args.nonEmpty) getopts.usage()
 
-      val target_version = Date.Format("uuuuMMdd")(Date.now())
+      val target_version = Date.Format.alt_date(Date.now())
       val target_dir = Path.explode("isabelle_fonts-" + target_version)
 
       val progress = new Console_Progress

--- a/src/Pure/General/date.scala	Fri May 07 16:49:08 2021 +0200
+++ b/src/Pure/General/date.scala	Fri May 07 21:03:20 2021 +0200
@@ -38,6 +38,7 @@
     val default: Format = Format("dd-MMM-uuuu HH:mm:ss xx")
     val date: Format = Format("dd-MMM-uuuu")
     val time: Format = Format("HH:mm:ss")
+    val alt_date: Format = Format("uuuuMMdd")
   }
 
   abstract class Format private